As a matter of experience, such a house is, taking all into consideration, the cheapest and most suitable type for any estate with an increase in output. Even at the outset there should be a separate room in which sorting and packing is undertaken. This is conveniently the lower room of a drying-house. The only stipulation to be made for a house with two storeys is that the floor of the upper room should be of an open pattern, so that the air may circulate right through the building. This is usually and very successfully attained by laying down wide slats of wood, with spaces of an inch or more between them. It is not advisable to have spaces wider than 11⁄2 inches, otherwise there is a certain amount of danger to the limbs of individuals who have to work or supervise in the building. In any case, it is very convenient to have pathways of planks running the whole length of the floor, so that the supervision is made more convenient. If this is done, there can be no objection to the custom of suspending the rubber of a lower chamber from the slats of the floor of the upper room. At present, in some drying-houses, this means of suspension is used, but no planks are laid down, and it becomes necessary to walk over the drying rubber. This is a detail, but it is one which does not make for the improvement of rubber, and the expenditure of a small sum would be sufficient to rectify the matter.
From every point of view, it would be desirable to have the floor of the packing-shed (or the packing-room in a combined house) raised from the ground, to a height of, say, 3 feet; or the height of a bullock-cart or motor-lorry. Not only is ventilation improved, but there would be a great saving in labour. Packed cases could be wheeled directly on a level with the cart or lorry.
A great many estates favour drying-houses of one storey. These are eminently suitable, provided that the site is suitable, and that the relative dimensions of the house are favourable to efficient ventilation. It is a common mistake to find buildings of which the breadth is out of proportion to the height. Obviously, if the height is not considerably in excess of the breadth, ventilation will be defective. For a single-storey drying-house, the maximum height should bear the ratio to the breadth of 3:2, and in a house of this type specially long pieces of crepe can be utilised. Naturally, in a house of two storeys, this factor is not likely to be neglected, and if the lower room is used for packing purposes the rate of drying should be rapid. Again, when a single-storey building is contemplated, it is well to make strict examination of local conditions. If the site is low-lying and surrounded by trees it will be clear that tall buildings are required, and that a house of more than one floor is to be preferred. Considerations of this nature would have prevented the erection of some dry-sheds which do not give a satisfactory rate of drying.
Ventilation.—No matter how many floors there may be in a drying-house, the greatest attention should be given to the question of ventilation. It is an elementary point in the study of ventilation problems that the best system of natural ventilation is obtained by admitting cool air near or through the floor and providing an exit for the warmer air at the highest point in the building. It is not often that such a rule is infringed in the ventilation of rubber drying-houses, but several of the older buildings erred in this respect. In a good modern house there is a space (about 2 feet in height) all round the base of the walls merely closed with expanded metal; this admits cool air. An exit for warm air is provided in the ridge of the roof by either ventilation chimneys or by a jack-roof. The latter is preferable, as it provides for a more free and uniform escape.
In some drying-houses, besides the ridge openings, the space along the eaves is left open. This would seem to be undesirable, as it provides for the entrance of outer air, which might combat the ascending warm air and so interfere with the natural upward currents. Provided that a jack-roof or other suitable openings have been installed, there is, therefore, no necessity for the existence of open spaces at the eaves, and they probably do more harm than good.
In the tropics, on days of sunshine, there must always be an upward current of air in well-designed houses. Temperatures of 105° F. are easily recorded in the ridge space of a building, while the temperature in the lower part of the house may be at least 15° F. lower. On the floor of an upper room a temperature of 90° F. is commonly noted, and in buildings with three storeys the usual day temperature of the top room is about or over 100° F. Even, therefore, when there is no trace of a breeze, there must be a displacement of air in an upward direction, though it may not be detected without tests being applied.
It is often asked whether a temperature of 100° F., such as is obtained in the upper room, is calculated to injure the quality of the rubber. There need be no fear on this ground; the experience of many estates goes to show not only that no harm results, but also that the drying of the rubber is expedited. There would seem to be no reason why crepe rubber should not be dried at a temperature of 100° F. It must be understood, however, that higher temperatures for crepe rubber are not recommended, as it has been proved that the rubber is affected. The fact becomes obvious with continued treatment at temperatures much above 100° F., for the rubber stretches and breaks across the support.
Windows.—Concerning the subject of window space in a drying-house, there has been much discussion at various times. Years ago it was common to find windows widely open with the sunshine streaming in. Naturally, tackiness developed in some of the rubber, and care was then taken to keep the windows closed. Thus the rooms were darkened and air excluded. There followed a period in which windows were fitted with ruby-coloured glass to keep out the actinic rays of the sun, which were responsible for tackiness, and excess of light, which was supposed to be responsible for the rapid oxidation of rubber. Unless special precautions were observed in the processes of coagulation and preparation, it was not proved that the exclusion of light prevented or lessened the natural oxidation of crepe rubber. Since the introduction of sodium bisulphite for the prevention of oxidation, there has been no cause to worry as to the possible effect of light, as no perceptible darkening of the rubber takes place. It follows, therefore, that no trouble need be taken to exclude light, although the necessity for excluding direct sunshine still exists. Windows may be left open as long as the sun does not reach them. This can usually be arranged in a drying-house by manipulating the windows at intervals during the day, so that those in the shady side of a building are always open, while those on the sunny side are always closed. If it is thought that this manipulation cannot be entrusted with success to the store coolies, the case may be met by having all windows constructed on the louvre pattern, so that, although the windows are closed all day, air and light are not excluded. Should it be desired to retain the existing type of windows, which open outwards, and to keep them open all day, a simple arrangement of ruby-coloured cloth on an outstanding wooden frame may be placed within the walls of the building, or the shutters of the windows may be hinged at the top to open outwards. Unless there is a pronounced breeze, or it is required to examine the rubber closely, there is no necessity to have windows open, except in the case of a house in which the bottom floor is used as a packing-room. The windows of this chamber may remain open during the day, to advantage in sorting and packing, and also to the proper ventilation of the building. Thus the direct rays of the sun are rendered harmless, while air and light are allowed to enter.
Hot-Air Drying-Houses.—Mention has already been made of the existence of a system of drying in which hot air is forced into a drying-house by means of a powerful fan. Provided that the temperature of the hot air could be so regulated as not to exceed 100° F., there would be merit in the system. Such matter of regulation could be solved by having a duct in the main air passage, through which cool air could be admitted in such proportion as to modify the temperature of the hot air. As the process is worked at present, the temperature attained is often well above 100° F., and there is a danger of thin crepe placed in this house over-night being found upon the floor in the morning. Unless the crepe is prepared thick and cut into fairly short lengths, it will not bear its own weight at higher temperatures; and if it is made thick, drying is impracticably prolonged. It is probable that, with a temperature of 100° F., and a steady current of air, average thin crepe would dry in such a drying-house within six or seven days. This would be an improvement upon the usual rate of drying in most factories, although several ordinary drying-houses are known in which thin crepe will dry naturally in that period.
Smoke-Houses.—No discussion of theoretical considerations regarding the process of smoke-curing will be attempted here. We are concerned only with the necessity for supplying a demand for smoke-cured sheet rubber. Broadly, the process is akin to the smoke-curing of herrings, and the objects are much the same—viz., (1) drying, (2) preservation—except that while herrings are only dried partially, rubber should be dried perfectly.